This invention relates to an expandable composition and blowing agent for producing low density thermoplastic foams and a process for producing an expanded thermoplastic foam product of low density using an expandable thermoplastic composition. In particular, this invention relates to use of a blended blowing agent for incorporating into a plasticized thermoplastic resin for foaming by extrusion.
Thermoplastic foam products can be produced by a wide variety of processes, of which extrusion is but one, that are in part responsible for the wide variety of foam products available today. Foams range in consistency from rigid materials suitable for structural use to flexible substances for soft cushions and packaging materials. These foams range in cellular formation from open or interconnecting-cell foams to closed or unicell foams. The cell structure may range from large to fine. Electrical, thermal, mechanical, and chemical properties can be varied within wide limits depending on the thermoplastic resin composition and the method chosen to create the foam. Foamed thermoplastics range in density anywhere from about 10 kg/m3 to over 1,000 kg/m3, although the latter perhaps more properly are called microcellular structures. True foams are considered to have a density of less than about 800 kg/m3.
Many methods have been developed for the manufacture of foamed thermoplastics, which generally can be classified into three groups: 1) methods for adding a gaseous xe2x80x9cblowing agentxe2x80x9d to the thermoplastic during processing, 2) methods for producing a gaseous blowing agent in the thermoplastic during processing, and 3) methods for forming a thermoplastic mass from granules to obtain a cellular structure. Similar blowing agents sometimes are used in the various methods to produce foams. However, the effectiveness of a particular blowing agent varies considerably depending on the thermoplastic resin composition, the method chosen, the process conditions, the additives used, and the product sought.
Blowing agents work by expanding a thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made. Bubbles of gas form around xe2x80x9cnucleation sitesxe2x80x9d and are expanded by heat or reduced pressure or by a process of chemical reaction in which a gas is evolved. A nucleation site is a small particle or conglomerate of small particles that promotes the formation of a gas bubble in the resin. Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution. However, the foam is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse. Additives may be incorporated into the resin and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
Methods for producing a blowing agent in situ usually involve a chemical reaction that evolves gas. Polyethylene, silicone, epoxy, and vinyl foams have all been produced by adding a substance that will produce a gaseous blowing agent chemically. For example, dinitroso compounds and hydrazides, which evolve nitrogen gas on decomposition, and bicarbonates, which evolve carbon dioxide, have been added to thermoplastic resins to produce foams.
Polystyrene foams often are produced by xe2x80x9cbead molding,xe2x80x9d in which partially expanded granules or beads are heated in a mold in the presence of a blowing agent to expand and fuse the particles into a rigid unicellular structure. A volatile organic compound or some other gaseous blowing agent is impregnated into the beads. Heat is applied and the pressure is released to cause the beads to expand and fuse.
There are several methods for adding a blowing agent to a thermoplastic resin during processing to produce a foam. Ureaformaldehyde and polyvinylformaldehyde foams have been produced by whipping air into a heated thermoplastic mass before it sets. Polyolefinic foams have been produced by introducing air or some other gas or volatile solvent into a heated thermoplastic polyolefin mass and then heating the mass or reducing pressure to expand the gas and form pores of a desirable size. One more specific method is to impregnate a thermoplastic resin with blowing agent under heat and pressure in a closed vessel. The pressure is released to expand the blowing agent to form xe2x80x9cprefoamed,xe2x80x9d or partially expanded, beads. Prefoamed beads usually are further expanded in an enclosed vessel such as a mold to produce a molded foam product, such as is discussed hereinabove.
Another more specific method, to which the invention claimed herein relates, is to mix the blowing agent with molten resin under pressure and then extrude the mixture through a forming die into a zone of reduced pressure. Shaped foams can be produced by extrusion foaming using a forming die of particular configuration. Plank, which can be cut to a desirable shape, and thin foam sheets are produced in this manner.
Extrusion foaming is a continuous process in which a plasticized thermoplastic resin is cooled and expanded when the resin and blowing agent are extruded into a zone of lower pressure. Mixing of blowing agent with polyethylene resin for extrusion foaming can take place in as little as few minutes or less because the resin is plasticized. Nucleating and stability control agents typically are used in extrusion foaming to control cell formation, diffusivity of the blowing agent, and stability of the foam.
Many of the halogenated hydrocarbons have been used for several years as blowing agents in the various methods for producing foams from thermoplastic resins. The halogenated hydrocarbons include the chlorofluorocarbons (xe2x80x9cCFCsxe2x80x9d) and hydrochlorofluorocarbons (xe2x80x9cHCFCsxe2x80x9d). CFCs and HCFCs are readily impregnable in thermoplastic resins and are readily expandable under relatively mild conditions. CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty. The pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs, and other halogenated hydrocarbons typically are either not flammable or are of low flammability, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity. However, governmental regulation is phasing out use of halogenated hydrocarbons because the halogenated hydrocarbons may be responsible for damage to the earth""s ozone layer.
Producers of thermoplastic foam products have been seeking alternatives to CFC and HCFC blowing agents for a number of years to reduce or eliminate altogether the amount of halogenated hydrocarbons used. A number of volatile organic compounds (VOCs) have been proposed, although many of these also are somewhat objectionable. VOCs include the light aliphatic hydrocarbons such as propane, n-butane, isobutane, butylene, isobutene, pentane, neopentane, and hexane, to name but a few. The diffusivity of VOCs can be many times faster than that of the halogenated hydrocarbons and harder to control. Foam collapse and stability problems have been encountered, although high quality foams can be produced using VOCs. Many VOCs are highly soluble in polyolefin resins and may be difficult or time-consuming to remove. VOCs typically are flammable, thus presenting handling problems and safety concerns.
Inert gases have also been proposed as blowing agents, although these sometimes do not provide acceptable results, especially for producing extruded foams. Inert gases include nitrogen, argon, xenon, krypton, helium, and carbon dioxide. Nitrogen and carbon dioxide, in particular, have the advantage of being inexpensive, readily available, and of not being flammable, and are not considered to be harmful to the earth""s ozone layer. However, inert gases usually are not as readily soluble in thermoplastic resins as CFCs, HCFCs, and VOCs, are of higher volatility, and do not reduce the viscosity of the resin. Lubricants often are added to the resin for extrusion foaming. The range of processing conditions for producing acceptable products is narrower than for CFCs, HCFCs, and VOCs. Mixing the resin with an inert blowing agent and keeping the inert blowing agent in the resin is more complicated than for CFCs, HCFCs, and VOCs. The surface texture of extruded foams sometimes is rough. Extruded low density foams and thick foams are difficult to achieve. The foam sometimes has poor stability and foam shrinkage can be uncontrollable.
Some blowing agent blends of VOCs and inert gases have been proposed for use in preparing extruded polyolefin foams. For example, Ealding U.S. Pat. No. 3,839,238 discloses extruded polyolefin foams prepared with a blowing agent system of a first blowing agent of saturated or unsaturated hydrocarbons and halogenated hydrocarbons including, among others, pentane, hexane, heptane, and octane, and a second blowing agent of carbon dioxide, nitrogen, air, methane, ethane, propane, and others. Foam density is disclosed as 0.05 to 0.5 g/cc (50 to 500 kg/M3), which is a relatively high density.
Lee U.S. Pat. No. 5,348,984 discloses a blowing agent blend for continuously extruded polyolefin foams of about 25 to 75% by weight carbon dioxide and about 75 to 25% hydrocarbon from the butanes and propane. Foam density is from about 20 to 40 kilograms per cubic meter (0.02 to 0.04 g/cc).
Lee U.S. Pat. No. 5,462,974 discloses a blowing agent blend for continuously extruded polyolefin foams of about 20 to 80% by weight carbon dioxide and about 80 to 20% hydrocarbon from the butanes and propane. Foam density is from 20 to 100 kg/M3. Less carbon dioxide is said to be useful if ethane is included in the blend.
The blowing agent blends of inert gas and VOC have been useful, but normally demand relatively slow extrusion rates.
Alternative blowing agents for extruded thermoplastic foams, processes for producing extruded foams using blowing agents, and thermoplastic resin compositions containing blowing agents are desirable that provide improvements in the production and characteristics of continuously extruded polyolefin foams.
The invention claimed herein relates to the use of a blowing agent for continuous extrusion foaming of thin sheets of relatively low density polyolefin foams at relatively high extrusion throughputs, acceptable pore structure, and with short aging characteristics. This invention is based on the recognition that initial foam dimensional stability as the foam is produced at high throughput can be controlled by use of a particular blowing agent blend in which small amounts of carbon dioxide are incorporated into C-2, C-3, and C-4 VOCs. While not wishing to be bound by theory, it is believed that small amounts of carbon dioxide promote faster exchange of air with blowing agent. In many, but not all, instances, the foam initially collapses slightly and then recovers at least about 95% of its as-extruded density. Foams produced in accordance with the invention are remarkably dimensionally stable, expanding and contracting less and aging about twice as fast as quality foams produced with VOC blowing agents not containing carbon dioxide. These properties are significant in the aging of roll stock, reducing thickness variations in the machine direction of the foam that can result from tightness in the rolls as the aging foam expands, and reducing the hazards inherent in handling and processing flammable VOCs.
Extruded thin foam sheets of from about 0.01 to 0.5 inches thick and having a low density of from about 10 to 60 kilograms per cubic meter can be produced by practice of the invention at extrusion throughputs rates of from above about 100 kg/hr to 250 kg/hr. Throughputs of 200 kg/hr are typical, although the precise rate is somewhat machine dependent and not always readily quantifiable for general application. Throughputs of 300, 400, and even 500 kg/hr should be possible with larger extruders, which normally have not yet been used for thin sheets of extruded low density foams, but may be enabled by the practice of the invention recited herein.
The foams expand and contract less than about 10 to 15% of their original as-extruded dimensions. Typically, these foams are aged for shipment within three days, normally achieving well less than 50% of the lowest explosive limit in air for the particular VOC used.
The blowing agent is a blended agent of carbon dioxide in an amount of from 1 to 25% by weight or less of the blowing agent. The balance is a hydrocarbon selected from among ethane, normal butane, isobutane, propane, or a mixture of any two or all of these hydrocarbons. Carbon dioxide concentrations by weight of blowing agent of 10, 15, and 20% are useful in the practice of the invention.
In a more specific embodiment, extruded thin foam sheets of low density polyethylene of nominal thickness of 0.05 to 0.150 inches and a density of from 15 to 30 kg/m3 are produced at the rate of from about 150 to 225 kg/hr using a blowing agent of propane or one of the butanes or mixtures thereof blended with carbon dioxide in an amount of from 10 to 20% by weight of the blowing agent. The foams are produced as roll stock and age as roll stock within 3 days to less than 30% of the lowest explosive limit in air for these VOCs. Dimensions of the foam during aging vary depending on the extrusion and aging conditions and the precise composition of the resin from which the foam is made. The foam may expand or contract, although usually less than for an all VOC blowing agent. Typically, if expanding, the foam may expand from the as-extruded dimensions by as little as 5% and typically by 8% or less. Tight roll conditions that may inhibit aging and can damage the foam through compression during storage are avoided.
Expandable resin compositions for producing extruded polyolefin foam products in accordance with the invention include a plasticized polyolefin resin mixed with a blended blowing agent as described. The expandable composition also normally contains a fatty ester aging or diffusivity modifier and a nucleation agent for controlling the size of the cells of the foam. Aging modifiers are thought to have a minimal impact on the diffusivity of carbon dioxide out of the cells of the foam. The presence of small amounts of carbon dioxide in the blowing agent, in accordance with the invention, tends to preclude tightness in the roll stock that is produced.
In more specific detail, the expandable composition may include a polyethylene resin, about 0.5 to 5 kilograms of glycerol monostearate per 100 kilograms of the resin, about 0.05 to 0.5 kilograms of zinc oxide per 100 kilograms of the resin, and about 1 to 20 kilograms of blowing agent per 100 kilograms of the resin, which blowing agent has a relatively low percentage of carbon dioxide mixed with either normal butane, isobutane, propane, or mixtures thereof.
Thus, a blowing agent of small amounts of CO2 mixed with ethane, or normal- or iso-butane or propane is provided that is capable of producing extruded, low density thin foam sheets at high extrusion rates comparable to VOCs, but with less instability after expansion and faster aging characteristics.